The Monitoring the Future project was created to assess the changing lifestyles, values, and preferences of American youth on a continuing basis. Trends in marijuana use and related attitudes were investigated, and alternative hypotheses about the linkage between attitudes and behaviors were examined. Data were obtained using questionnaires administered to nationwide samples of approximately 17,000 high school seniors annually from 1976 through 1985. Trend data showed that perceived risks and personal disapproval increased steadily from 1978 onward, whereas actual use of marijuana reached peak levels in 1978-79 and then declined during the early 1980s. Subgroup trend analyses revealed that when attitudes were held constant there was no decline in rate of marijuana use, supporting the hypothesis that the overall decline found in the total samples was attributable to changes in perceived risk and disapproval. Reversing the subgroup trend analysis procedure, by holding constant the levels of use, provided no support for the alternative hypothesis that changes in use caused the trends in attitudes. The findings are useful theoretically because they indicate that in this area attitudes seem to shape behaviors, rather than the reverse. For those involved in prevention efforts, the findings suggest that realistic information about risks and consequences can play an important role in reducing the demand for drugs. (Author/NB)